The present invention is directed to methods and devices for atomizing liquids. More specifically, the liquids are atomized at the exit of an elongated, small diameter tube or a small internal surface area chamber, with an optional heating device for directly heating the liquid within the tube or chamber. The atomization devices are useful in many applications including, but are not limited to: flame and plasma based atomic spectroscopy, nano-powder production; particle/droplet seeding for laser-based flow diagnostics; spray drying for the production of fine powders; nebulizers for inhalation in delivery of medication and for atomizing fuel for use in combustion chambers.
Atomizers are already used in many applications for producing finely divided aerosols with uniform droplet size distribution. While some of the prior art atomizers are at least partially effective, there is still a need for an atomizer that can produce a finely atomized spray with controlled and uniform droplet size distribution. The article on pages 2745-2749 of Analytical Chemistry 1990-62, entitled xe2x80x9cConversion of an Ultrasonic Humidifier to a Continuous-Type Ultrasonic Nebulizer for Atomic Spectrometryxe2x80x9d and authored by Clifford et al., discusses the most commonly used solution nebulizers for atomic spectrometry. U.S. Pat. No. 4,582,731 issued on Apr. 15, 1986 to Smith, discloses a supercritical fluid molecular spray film deposition and powder formation method. The generation of particles and seeding in laser velocimetry is described by James F. Meyers in the von Karman Institute for fluid dynamics, lecture series 1991-08. This reference also discusses the increase in accuracy of laser measurements when uniform size particles are used. A nebulizer device for the delivery of medication is described in U.S. Pat. No. 5,511,726 issued to Greenspan et al. on Apr. 30, 1996. The device uses a piezo-electric crystal and control circuit to apply a voltage to a sprayed solution.
In addition to the above prior art atomizers, various methods and apparatus for preheating or atomizing fuel have been developed over recent years. While some of these devices are partially effective, there is still a need for an atomizer that can completely vaporize the fuel, as well as raise the temperature of the fuel to avoid condensation downstream of the atomizer. This is particularly useful during the cold start and warm-up cycle of an internal combustion engine. After an engine has been allowed to cool significantly below operating temperature (as little as several minutes after shutting it off, depending on the weather) and is then started, the fuel entering the combustion chamber is often in vapor, large droplet and liquid form. Large portions of the fuel that is in droplet or liquid form does not burn completely. This results in reduced engine efficiency (used but unburned fuel) and an increase in the production of unburned hydrocarbons. Not only is the engine not hot enough to effectively burn the non-atomized fuel, but the after-treatment (i.e. catalytic converter) is non-operational during this heavy pollution producing period of operation. In fact, seventy to eighty percent of all hydrocarbon emissions are generated prior to the catalytic converter coming on line. By decreasing the size of the fuel droplets and increasing the vaporization of the fuel entering the combustion chamber, the percentage of the fuel that is burned is increased, thereby producing more heat and reducing the time needed to bring the engine and catalytic converter to operating temperature.
U.S. Pat. No. 4,011,843 issued to Feuerman on Mar. 15, 1977, discusses vaporizing fuel for use in internal combustion engines. A spray valve for a fuel injected, IC engine is taught in U.S. Pat. No. 4,898,142, issued on Feb. 6, 1990 to Van Wechem et al. U.S. Pat. No. 5,118,451, issued on Jun. 2, 1992 to Lambert, Sr. et al. is drawn to another fuel vaporization device. In U.S. Pat. No. 5,609,297, issued on Mar. 11, 1997 to Gladigow et al., several embodiments of a fuel atomization device are described. A fuel injector with an internal heater is disclosed in U.S. Pat. No. 5,758,826, issued on Jun. 2, 1998 to Nines. U.S. Pat. No. 5,778,860, issued on Jul. 14, 1998 to Garcia, teaches a fuel vaporization system. SAE Technical Paper Series #900261 entitled xe2x80x9cThe Effect of Atomization of Fuel Injectors on Engine Performancexe2x80x9d, and written by Kashiwaya et al., discusses the use of injectors with swirl patterns. SAE Technical Paper Series #970040 entitled xe2x80x9cFuel Injection Strategies to Minimize Cold-Start HC Emissionsxe2x80x9d, and written by Fisher et al., describes the effect of changing fuel injector and control parameters on cold-start emission levels. SAE Technical Paper Series #1999-01-0792 entitled xe2x80x9cAn Internally Heated Tip Injector to Reduce HC Emissions During Cold-Startxe2x80x9d, and written by Zimmermann et al., is drawn toward measuring the effect of internally heated fuel injectors on HC emissions prior to an engine reaching operating temperature.
The present invention involves controlled atomization of liquids for various applications such as particle/droplet seeding for laser-based measurements of flow velocity, temperature, and concentration; flame and plasma based atomic spectroscopy; nano-powder production; spray drying for generation of uniform-size powder; chemical processing (i.e. phase transformation, dispersions, catalysis, and fuel reformation); nebulizers for inhalation applications and for atomizing fuel for use in combustion chambers. In these and other atomizer applications the control of droplet and/or particle size and uniformity is critical. In some applications extremely small droplets are preferred (less than a micron), while in others, droplet diameters on the scale of several microns are required. However, most of the above-mentioned applications require finely dispersed spray with droplets sufficiently uniform in size (i.e. mono-dispersed). Other applications desire very fine droplets for increased surface area interaction for improved reactions, thermal and chemical equilibrium rates, phase transformations and uniformity. The atomizer of the present invention has the flexibility of forming droplets with controlled size, wherein not only the size of the average droplet can be adjusted, but the range of sizes may be adjusted as well. The methods of using the atomizer are described below with reference to the specific application.
The use of laser technology in the measurement community has increased significantly over the past few decades and continues to gain acceptance as new and improving technology evolves. An advantage of laser technology is that the light is non-intrusive and non-destructive and the condensed intensity inherent to laser beams allows for very accurate sensing of very small particles making very small changes. One such application is the use of laser beams to make velocity measurements, and is known as laser Doppler velocimitry (LDV). The laser beam is directed at moving particles, and the velocity of the particles is measured. Often, this type of measurement is used to study the velocity characteristics of a gas flow, such as air, through a duct. To provide an object for the laser beam to be reflected by in air and other gases, one must introduce some medium that is large enough to be illuminated. In demonstrations, this is typically accomplished with smoke. However, measurements such as LDV typically require a slightly larger particle in the sub-micron to several micron range. In addition to the size sensitivity, the reflecting medium can change the parameters that are being measured as well. To study the velocity characteristics of a gas flow, one must xe2x80x98seedxe2x80x99 the gas flow with enough sub-micron to several micron particles to make measurements possible, while at the same time not affecting or degrading the gas flow. This seeding requirement is often the most difficult requirement to achieve for accurate and reliable LDV measurements. Currently available atomization devices are used for seeding but typically do not yield the desired performance. A combination of low volume and inadequate atomization result in too few measurements in a desired period of time. For instance, to make high-speed measurements one must acquire several thousand measurements over the course of one minute. These measurements can then be averaged to provide accurate results.
The present invention comprises methods and devices capable of generating sprays with small, uniformly-sized droplets by superheated atomization. This atomizer was tested as a particle-seeding device for LDV measurements, and was shown to provide significant improvements in number of counts per minute and signal-to-noise ratios. The improvement is caused by the atomizer""s superb ability to finely atomize liquid in precise doses by operating on a heat based atomization method as opposed to air induced atomization. In the superheated atomization, a pressurized liquid is raised to an elevated temperature in the atomization nozzle, resulting in a heated spray that is more resistant to re-condensation. This resistance proves beneficial as the atomized spray propagates to the measurement section without re-condensing. The improvements in particle seeding for LDV systems that are achieved using the present invention, can also be expected to improve measurements in other systems that use particle seeding, such as wind tunnel testing. To this end, the atomizer of the present invention was tested for atomizing a liquid with suspended particles. The particles used in the test were titanium dioxide in the 3-5 micron size range. The atomizer achieved excellent atomization and thereby uniform entrapment of the titanium dioxide particles in the air stream in a neutrally buoyant sense. These test results indicate that the atomizer can be used as a smoke generation device for wind tunnel testing. A steady, dense, repeatable and controlled volume smoke stream was easily producible with the atomizer.
It has been demonstrated that the atomizer of the present invention can achieve data rates that are two-orders of magnitude higher than data rates obtainable with conventional particle seeders. By optimizing fluid and gas flow rates, and the power input to the atomizer, further improvements in sensitivity can be obtained for a wide range of materials and particles. Furthermore, the use of the atomizer as a particle seeder for flow measurements will allow precise, on-the-fly control of the droplet size and density. Currently, solid seeding particles with fixed size distribution have to be replaced between the runs with different flow parameters requiring different particle sizes. In short, the atomizer can control droplet size and spatial distribution and optimize signal levels while reducing the particle interactions with the flow field.
Another application of the atomizer is in the field of flame and plasma based elemental analysis. In U.S. Pat. No. 5,997,956 issued Dec. 7, 1999 to Hunt et al., and entitled xe2x80x9cCHEMICAL VAPOR DEPOSITION AND POWDER FORMATION USING THERMAL SPRAY WITH NEAR SUPERCRITICAL AND SUPERCRITICAL FLUID SOLUTIONSxe2x80x9d, one embodiment of the atomizer is used in conjunction with the CCVD process. In this coating process, precursors are dissolved in a solvent acting as the combustible fuel. This solution is atomized to form sub-micron droplets that are carried by an oxygen stream to the flame where they are combusted. The heat from the flame provides the energy required to evaporate the droplets and for the precursors to react and to deposit on the substrates. By modifying the CCVD system, measurements of the optical emission from excited species in the flame can be made, and these measurements can be analyzed for trace analysis. One such application includes flame based Atomic Emission (AE) spectroscopy. Two of the most commonly used analytical techniques for elemental analysis are Atomic Absorption spectroscopy (AA) and Ion Cyclotron Plasma Atomic Emission spectroscopy (ICP AE). AA instruments are relatively inexpensive but have somewhat limited sensitivity (detection limit). ICP AE has a much greater sensitivity than AA, but is much more expensive. It has been demonstrated that the present atomizer can produce flames for AE spectroscopy such that measurements are of sensitivities comparable to the state of the art AA results. This sensitivity was achieved without major modifications to the existing CCVD setup, and the resulting system was far from optimum. Through optimization of fluid and gas flow rates, atomizer settings, flame positioning, signal integration, and optics settings, significant improvements to sensitivity can be obtained. The atomizer of the present invention will achieve ICP AE quality results with an instrument that could very well sell in the price range of an AA. In atomic spectrometry, the efficient nebulization of organic solutions and the reduction of the mean drop size result in an increase of measurement sensitivity and analyte transport efficiency. Furthermore, the kinetics of the vaporization process that occurs in the measurement chamber are determined by the fraction of large aerosols present in the chamber, which is directly related to the mean diameter of the primary aerosol produced by the nebulizer.
The potential for using this atomization device in flame emission spectroscopy was put though preliminary testing using toluene solutions of known sodium concentrations. A fiber optic spectrometer was used to observe the intensity of the sodium xe2x80x9cDxe2x80x9d lines for solutions of different sodium concentration. The lowest concentration tested (1 ppm) was easily detectable, with the sodium lines having signal to noise ratios visually estimated to be well above 10:1 even at such low concentration. The system was found to be very sensitive to small changes due to factors such as spray uniformity, nozzle position, etc. The system of the present invention has a sensitivity that could rival ICP detection limits at a fraction of the cost in instrumentation. Further this system can use hydrocarbon solutions. To reduce background solvent peaks, the current invention can be used in a ICP system or with H-O flame. Other plasmas can also be used, such as microwave and electric arch plasmas. In such plasma systems improved sensitivity will result using the present invention from finer atomization and little or no dilution from atomizing or propagation gases.
The atomizer is also useful in the production of nano-powders (1-100 nm). There are many existing technologies for the production of fine powders, including chemical vapor condensation, flame-based condensation, and plasma processing. These techniques are useful for production of homogeneous and small-sized powder, but are very energy intensive and therefore expensive. Compared to these techniques, the present invention offers significant processing cost reduction. Furthermore, the atomizer process will also enable numerous nano-powder compositions that cannot be formed by conventional techniques. In liquid combustion vapor condensation (LCVC), low-cost, environmentally friendly, metal-bearing reagents are dissolved in solvents that also serve as combustible fuel. Using the atomizer of the present invention, this solution is atomized to form sub-micron droplets, which are then combusted in a torch, forming a vapor. The condensable species thus formed nucleate homogeneously as aerosol nano-powders that are then collected in dispersion media or on a solid collector. Premixed precursor solutions allow great versatility in synthesizing a wide variety of nano-powder compounds of very uniform size and composition. The LCVC method can produce nano-powders that are collected as colloidal dispersions, which is a convenient form for handling and subsequent processing. Applications that can benefit from the production of these nano-powders include near net shape ceramics, powder coating, and rheological fluids. Other applications of these high quality, multi-component nano-scale powders include electronic, optical, magnetic, mechanical and catalytic applications. For gas phase chemical processing, powders or nano-powders can be introduced to be reacted or act as a catalyst. Use of the atomizer with LCVC results in a simple and economical manufacturing process for a variety of advanced nano-phase powders.
Yet another useful application of the present atomizer is as a novel nebulizer for generating small-droplet sprays. The atomizer enables very fine atomization and vaporization of the liquid solvents and fuels, and complete and high-speed control of atomization, while utilizing an innovative combination of simple, robust components with modest power requirements. These features are useful for sample introduction in flame and inductively-coupled plasma atomic spectroscopy, as explained above, as well as many other equally important processes, including mass and atomic emission spectrometry, drug delivery, and fuel analysis and injection. In another chemical processing application, hazardous materials can be more finely and uniformly divided, to enable safer and more complete decomposition processing via thermal, plasma, flame or other reactors.
Spray drying technology is used in the generation of small-sized particles. The atomizer enables very fine atomization and vaporization of the liquid solvents and complete control of the degree of atomization. These features are useful in spray drying processes for production of pharmaceutical dry powders and atomization of suspensions and slurries for food and chemical products. This invention can also provide more efficient production of polymer powders with precise particle size. Spray-drying processes involve transforming a liquid into a dry powder particle. This is achieved by atomizing the fluid into a drying chamber, where the liquid droplets are passed through a hot-air stream and transformed into solid particles through a mechanism controlled by local heat and mass transfer conditions. These particles are then collected and stored for future use. The main objective of the atomizer is to produce a spray of high surface-to-mass ratio, droplets that can uniformly and quickly evaporate the water or other solvents. This step in the spray-drying process defines the primary droplet size and therefore significantly impacts the quality of the produced powder. In applications such as pulmonary delivery of protein and peptide therapeutics, the drug must be delivered in small sized particles to prevent exhalation or deposition on the upper airway. Other applications of the spray drying technique using the atomizer of the present invention include tile and electronic press powders that play an important role in the industrial development of high performance (advanced) ceramics. The ability to meet particle size distribution requirements, produce a spherical particle form, and handle abrasive feedstocks is an important reason for the widespread use of spray dryers in the ceramic industries. Spray dryers for the chemical industries also produce a variety of powdered, granulated and agglomerated products in systems that minimize formation of gaseous, particulate and liquid effluents. High efficiency scrubber systems and high performance bag filters prevent powder emission, while recycle systems eliminate problems of handling solvents, product toxicity, and fire explosion risks. Food products that are in powder or agglomerate form such as coffee/coffee substitutes, food colors, maltodextrine, soup mixes, spices/herb extracts, tea, tomato, vegetable protein, can be formed using spray drying. This application of the atomizer is useful as the formation of these heat sensitive products requires careful selection of the system and operation to maintain high nutritive and quality powders of precise specification.
The present invention also involves the atomization of fuels for delivery to combustion chambers to enhance the burning of these fuels, thereby increasing the fuel and thermal efficiency while reducing the amount of unburned hydrocarbon pollutants produced by the combustion. The methods and apparatus described herein are particularly beneficial when used to provide atomized fuel during the start and warm-up cycles of internal combustion engine operation, when fuel consumption and pollutant production are at their highest levels (it should be understood, however, that the invention is not intended to be limited to use with any particular fuel or combustion chamber, but has a wide range of useful applications). When the engine is operated prior to reaching its normal operating temperature (an action that is inherent to all engines that must be started), the ambient temperature internal surfaces of the engine (particularly the intake path) prohibit the fuel vaporization process, and even induce wetting of these surfaces. The non-vapor phase of the fuel does not burn, so a reduction in the vaporization of the fuel results in an increase in fuel consumption and the production of pollutants (namely unburned fuel), as well as a decrease in specific power efficiency. By routing the fuel through a small bore tube or chamber and rapidly heating the fuel in the tube, the present invention produces a finely atomized, heated fuel with droplets in the sub-micron to micron range. This highly atomized fuel bums thoroughly enough to reduce cold-start and warm-up emission levels to levels similar to those produced after the engine has reached operating temperature.
By providing heated, highly atomized fuel, the fuel atomizer of the present invention avoids wetting and puddling on the fuel injector, throttle body, intake walls, valves, valve stems, valve seats, valve relief, cylinder wall, cylinder head, spark plug, spark plug threads, piston lands, piston crevices, piston faces, piston rings and other internal engine surfaces. The liquid fuel that collects on these surfaces, not only increases fuel consumption by not burning but also acts as a heat sink, thereby prohibiting heat transfer to the engine and increasing engine warm-up time. The atomizer heats the fuel by directly contacting the fuel with the heating element at the point of injecting the fuel into the engine. The atomizer can be used to inject fuel in several different locations within the engine, either as a supplemental injector (i.e. cold start injector), or as the primary fuel injector. Fuel can be delivered into the intake manifold, port or directly into the combustion chamber, pre-chamber or stratification chamber. In addition, the atomizer can be configured to operate in any combination of these locations as a central port injector or as an individual component of a multi-port injection system, and either as a complete, variable flow, fuel delivery system or as a supplemental cold-start fuel injection system.
It should be noted that while the examples and data herein are predominately drawn toward gasoline burning, internal combustion engines, the atomizer is fully capable of producing atomized fuel for use with any combustion device and with other fuels as well. Examples of fuels include gasoline, diesel, kerosene, bio-fuels, heating oil or gas, A1, JP-5, and JP-8. Examples of useful applications include two and four stroke internal combustion engines, furnaces, turbines and heaters. There are an unlimited number of fuels and applications to which the present invention can be applied, and therefore it is not intended to limit the fuel atomizer to any particular application. To this end, the terms xe2x80x9ccombustion chamberxe2x80x9d and xe2x80x9cfuelxe2x80x9d have been used herein to refer to any device that burns fuel, and can benefit from increased atomization of that fuel. As one of the most advantageous uses of the fuel atomizer embodiments of the present invention, however, is to reduce emissions and fuel consumption during start-up of internal combustion engines, this application has been the first to be investigated.
The atomizer of the present invention can be formed as several different embodiments. In the basic embodiment the atomizer is a heated tube or chamber. The method of heating the tube can be chosen from a number of different methods, including, but not limited to: direct electrical resistive heating (using a resistive tube or internal heating element); conductive heating (placing the tube in a block of material and then heating the block by a cartridge heater), by passing heated fluids over or through the block or other heating means); radiant heating using laser, infrared, microwaves or other radiant energy source(s); hot gases or liquids (oils, water, glycol), flames directed about the tube; or any combination of these and other known heating methods capable of achieving the required liquid temperature. Electrically resistive heating is preferred, as this provides a large range of controllable heating in a relatively small space. In the basic electrically heated embodiment, an electrically conductive/resistive tube or chamber is used. The term xe2x80x9ctubexe2x80x9d is intended to indicate a structure having an internal surface area that is small relative to the length of the structure. This can be better represented by defining the length to characteristic internal width (CIW) ratio. The CIW can be expressed as the square root of the average cross-sectional, internal area of the chamber. For example, a uniform square tube with 3 mm sides would have an average cross-sectional area of 9 mm2, and a CIW of 3 mm. If this tube were 12 mm long, the length to CIW ratio would be 4. While some applications can operate with length to CIW ratios as little as 1, most applications require length to CIW ratios of 50 to 100 for proper atomization of the liquid to occur. Higher CIW ratios normally provide finer and more uniform droplets. CIW ratios even above 1000 are very useful. Higher CIW ratios increase the back pressure which can be helpful in some applications or limiting in others. The actual internal cross-sectional area and length required is dependent on the required liquid flow for the particular application. For a liquid flow of 25 ml/min., one may expect a defined ratio of 100. The outlet of the atomization device includes one or more liquid ports for delivering the atomized liquid to the required location, which is dependent on the particular application (smoke chamber, in-take manifold, etc.). In electrically heated embodiments, an electrode is attached either directly to an end of the device, to the connection fittings or to any conductive object in electrical contact with the heating element portion of the atomizer. A voltage is applied across the electrodes sending electrical current through the material around the chamber, (or an internal heating element), to thereby heat the material that is in direct contact with the liquid inside of the tube. As the liquid propagates through the device, its temperature increases rapidly to a level above the boiling temperature of the liquid at atmospheric conditions. However, since the liquid is kept at an elevated pressure, it remains in the liquid phase throughout the heating chamber. The pumping pressure used to drive liquid through the device acts to increase the boiling temperature of the liquid, thus allowing it to reach temperatures much higher than the boiling temperature of atmospheric liquid. Upon exiting the device, the heated liquid is in a metastable state and it rapidly expands in the surrounding atmospheric or reduced-pressure environment. This rapid expansion of hot liquid results in extremely fine atomization of the liquid. The electrical power applied in such a manner is adjustable to calibrate the heating of the tube so as to tailor the atomization to the particular liquid and/or application. Furthermore, this adjustment can be made xe2x80x9con-the-flyxe2x80x9d to allow controlled atomization of different liquids, and/or combinations of liquids that have different atomization requirements, or to adjust the mean particle size and size distribution needed for the particular application. While the basic embodiment illustrated herein has a straight, circular cross-sectional configuration, other chamber shapes, such as coiled, bent, twisted or others can be used to suit the application and space requirements. It is also not required that the tube or chamber be circular in cross-section, but can be square, triangular, elliptical, etc. The atomizer may be made of a wide range of different materials depending on the desired resistivity, strength, thermal characteristics, etc.
In addition to the basic embodiment, several variations are disclosed herein. A further embodiment has a tube or body that is constructed of a non-electrically conductive material such as ceramic or glass. A central, heating wire or element extends along the longitudinal axis of the ceramic tube, thereby contacting and heating the liquid as it flows through the tube and about the heating device. The ceramic tube provides electrical and thermal insulation for the heating element and also provides structural strength for the heating wire or element. Other embodiments include a spirally shaped heating wire that extends along the inside surface of the chamber from one end to the other or within any section of the interior. Such a configuration provides additional surface area of heating element per length of chamber, as may be required for high flow rates or increased heating. One advantage of the ceramic or insulated chamber embodiment is the ability to use a wire heating element made of more efficient, yet potentially less robust material. Furthermore, the insulating material of the atomizer could be electrically as well as thermally insulating, thereby reducing heat transfer to surrounding components and increasing efficiency. As with the first embodiment, the delivery end of the ceramic tube can include one or more liquid delivery ports.
The above-described embodiments can also incorporate additional modifications designed to maximize the overall efficiency of the atomization device and the particular application. Any of the above atomizers could comprise multiple, series or parallel tubes. These tubes could be of alternating sizes, shapes, or cross-sections depending on the combustion chamber requirements or other factors. For example, the tubes or chambers could be of consecutively smaller diameter, with initial tubes or chambers of coiled configuration and a final tube with a straight configuration for targeting the liquid upon exiting the atomizer. The specific combination of tubes having similar or different diameters, cross-sections, lengths, thicknesses, configurations (coiled, bent, spiral, multi-tube twisted, etc.) and nozzle sizes would depend on the application.
Further modifications include the addition of materials on the outer surface of the atomizer. These materials could be integrated with the main tube and be in the form of increased tube thickness, or they may be in the form of a sleeve or sleeves of different materials (such as positive temperature coefficient (PTC) materials) coated, bonded, or otherwise attached to the outer surface of the atomizer. The function of these materials could be any combination of adding strength to the overall atomizer, acting as a heat sink or reservoir for temperature stabilization, and/or thermal/electrical insulation. The overall shape and size of the atomizer would be optimized for the application.
Many different materials may be used to produce the various components of the liquid atomizer of the present invention. The heating element (wire, tube, etc) can be any thermally/electrically conductive/resistive material that is not degraded by the liquid or the required heat and pressure. PTC material may be used for maintaining a specific temperature, as is well known in the art. In the electrically heated tube embodiments, stainless steel has had satisfactory results, in terms of conductivity, heat transfer, strength and liquid resistance. In electrically insulated tube embodiments, the tube can be made of any electrically insulating material that is not sensitive to the liquid atomized. Heat loss can be minimized by using a thermally insulating material or air gap and/or increasing the wall thickness of the tube.
A number of atomizer power control methods may be employed to control the temperature and pressure of the liquid, thereby changing the mean droplet size, droplet size distribution and other application specific factors. In some applications, partial boiling of the liquid may be preferred. As the temperature of the liquid increases, droplet size decreases and the amount of gas and vapor state of the liquid increases. Depending on the application, the wt % of these stable gases and vapors may be 1%, 5%, 10%, 20% or even as high as 40% of the total fluid exiting the chamber. An optimal thermodynamic state of the liquid exiting the nozzle (temperature and pressure) is selected on the premises of these factors. The level of atomization and liquid flow rate and properties, directly dictates the power requirement of the device. As with prior art devices, the required power level is determined by input-output comparative analysis, power to device, and level of atomization as determined by mean droplet size and uniformity per liquid type, as well as the heating method, materials used to form the atomizer, heat transfer rate and other factors. The device is capable of operating over a large range of power settings. Very low power settings result in average atomization and droplets in the range of 20-100 xcexcm. However, high power levels result in sub-micron atomization. As previously described, the power setting can be adjusted during operation of the atomizer by simply changing the voltage applied to the material of the atomizer or the heating element. The power setting results in a particular maximum temperature of the liquid within the chamber (usually just as the liquid exits the chamber). This maximum temperature may be sustained for a short length of time from fractions of a millisecond to 0.01 or 0.1 second, or may be maintained for one second, 10 seconds or even as long as one minute, depending on the atomization properties of the liquid as well as the flow rate through the chamber. The pressure of the liquid entering the chamber is also controlled (by the upstream pump or pressure regulator), to provide a specific pressure drop between the entrance and exit of the chamber. A 10 psi drop may be adequate; however, 50 psi, 100 psi or even a 300 psi pressure drop may be required. Variation of CIW and CIW to length ratios can be used to realize the desired flow rate and desired back pressure. Some of the liquid atomization properties that determine the required temperatures and pressures include liquid and gas temperature and pressure relationships (such as the boiling point), surface tension, viscosity, and level and size of any suspended solids that may be in the liquid.
Accordingly, it is a first object of the invention to provide a controllable liquid atomization method for producing specific mean droplet sizes and droplet size distributions, depending on the specific application.